The Error in Linear Interpolation at the Vertices of a Simplex

A formula for the error in multivariate quasi-interpolation which reproduces the linear polynomials is given. From it sharp pointwise $L_\infty$-bounds for the error in linear interpolation (interpolation by linear polynomials) to function values at the vertices of a simplex are obtained. The corresponding 'envelope theorem' giving the optimal recovery of functions is discussed.